1. Field
This patent specification relates to improved scintillator based radiation detection. More particularly, this patent specification relates to methods and systems for using improved energy calibration, resolution monitoring, and count rate calibration using intrinsic radiation sources.
2. Background
Scintillation detectors featuring a scintillator crystal and a photodetector (for example a PMT tube) are widely used different industries, and in particular in the field of oilfield services. A common problem in the use of scintillation detectors for nuclear spectroscopy or similar energy sensitive measurements is that the detector response function changes for example with changing environmental conditions. Typically, the sensitivity of the photodetector element will vary with time (drift) and with changing environmental conditions such as temperature and magnetic fields.
Scintillator materials are widely used to build detectors for measuring X-ray and γ-radiation. Dense materials with high atomic numbers are preferred to measure γ-rays, since the stopping power of the materials increases with these parameters and thus the size of the detector can be reduced without loss of sensitivity. However, many of the heavier scintillator materials have an intrinsic background radioactivity due to the presence of radioactive isotopes in the heavier elements of the crystal matrix. In particular Lutetium has been found to be a valuable constituent in scintillator materials, but suffers from the presence of a radioactive isotope. In large detectors this background count rate might contribute significantly to the maximal achievable count rate and thus negatively affect the precision and accuracy of the measurement. For example lutetium oxyorthosilicate (LSO) has been established as a useful scintillator for medical imaging, but its intrinsic radioactivity affects the count rate in large scintillator crystals. More recently LuAP and LuAG have been used as matrix materials for scintillators. Typical intrinsic count rates for material containing a large fraction of Lu are around a few hundred counts per second per cubic centimeter (cm-3s−1). For example a 2″×4″ crystal contains about 200 cm3 of material, so that the count rate reaches around 50,000 s−1. This is about 5-10% of the count rate capability of a fast conventional detector and thus creates a loss in statistical precision of several %. Intrinsic radioactivity is therefore conventionally regarded as a disturbance.
Dead time can be measured by placing a pulse generator (or short pulser) signal into the spectrum where it can reasonably well be distinguished from count rate related to gamma (or particle) counting. The dead time of the system can then be determined from measuring the total number of counts. An external pulser can either be an electronic component that feeds a reference signal of known and stable amplitude into the electronics of the scintillation detector or it is based on pulsed light sources where the reference optical pulse is fed somewhere into the optical system of the scintillation detector and is detected through the photon detection system.
Similarly, pileup can be measured using random pulsers with the caveat that it is difficult to create artificial events that match real radiation in timing signature and randomness of timing.
Alternatively a monoenergetic external source can be used as a reference to create a signal of known count rate in the measured spectrum. However, using an external source can lead to problems with the absolute count rate, if the geometry and the efficiency of the detector were not well known. Additionally, count rate changes can occur due to changes for example in geometry for example due to temperature expansion between external reference source and crystal.